This invention relates to a brake pressure regulator with a by-pass for a vehicle brake system including a first brake circuit associated with the front wheels and a second brake circuit associated with the rear wheels. The wheel cylinders of the first brake circuit are connected to a first pressure chamber of a tandem master cylinder and the wheel cylinders of the second brake circuit are connected to a regulator outlet chamber of the brake pressure regulator and a second pressure chamber of the tandem master cylinder is connected to a regulator inlet chamber of the brake pressure regulator. The pressure regulator includes a control piston and a valve member is provided in a first pressure fluid line between inlet chamber and outlet chamber. The control piston is subjected to the pressure of the outlet chamber and to the pressure of the inlet chamber and a second pressure fluid line is arranged between the inlet chamber and the outlet chamber to by-pass the control piston and second pressure fluid line is normally sealed with respect to the pressure of the second brake circuit by means of a closure piston acted upon by a spring force and having the pressure of the first brake circuit acting thereon.
A brake pressure regulator of the aforementioned type is known from German patent DE-AS No. 2,163,635. An axial bore in the housing of the brake pressure regulator accommodates an insert having several passageways and providing a tight isolation between inlet and outlet. A bore connecting directly with the outlet terminates towards the inlet side in a cup-type body into which a closure plug is sealingly inserted, with the plug being held in the closing position by a spring and being acted upon by the pressure of the first brake circuit in opposition to the outlet pressure. The housing of the brake force regulator accommodates a further bore connecting with the inlet and terminating in the regulator inlet chamber of the pressure regulator arranged at right angles to the insert. The regulator outlet chamber communicates direct with the outlet through the passageways provided in the insert. With the first brake circuit intact, the closure plug remains tight in the cup-type body, and pressure fluid is allowed to flow from the inlet to the outlet only via the controlled member. However, in the event of failure of the first brake circuit, the controlled pressure in the passageways of the insert will push the closure plug out of the cup-type body against the force of the spring, thereby opening the direct connection from the inlet to the outlet which by-passes the controlled member. This design is, however, particularly complicated and, consequently, expensive to manufacture. In additon, the arrangement of the pressure regulator at right angles to the housing of the brake force regulator necessitates unfavorably much space which is not in line with today's demands for minimum space requirements.
German Patent DE-OS No. 1,928,647 shows another version of a brake pressure regulator with by-pass. Therein, a conventionally used pressure regulator is by-passed by a power system including a by-pass valve. This valve comprises a piston sealed to and slidable in a housing, with the piston head being of a diameter smaller than that of the remaining piston and carrying a sealing plate. The bore accommodating the piston lin the valve housing is divided into two pressure chambers by the piston. The first pressure chamber is pressurized by the pressure of the first brake circuit so that the piston head is urged into abutment with the inlet connected to the master cylinder and arranged axially to the piston, thereby closing the inlet bore of the first pressure chamber. The inlet bore terminates in a tapering truncated cone so that only the area of the inlet that is necessary to provide a seal abuts the sealing plate of the piston to thereby isolate the second pressure chamber and the wheel cylinders of the controlled brake circuit, which wheel cylinders connect with the second pressure chamber through an outlet bore, from the inlet. In the event of failure of the first brake circuit, the pressure in the second pressure chamber and the pressure in the inlet will cause an abrupt displacement of the piston and open the connection from the master brake cylinder to the wheel cylinders, which connection by-passes the controlled member. Though the separate arrangement of the components may permit a better utilization of the space available, the amount of pressure pipes involved is substantial. This piping system requires the added provision of sealing screw connections and tees resulting in increased susceptibility to trouble in the brake system and higher costs.